I found a tip on a carseat safety board for easing leg numbness for little ones in carseats, especially on long trips. It is very simple, cheap, and safe.
Take a swim noodle with a hole through the center, measure the front width of the carseat, and cut the noodle to that length (serrated knife works great). Then simply thread a rope, ribbon, or strap through the center, put it down around the carseat base, and adjust until it's at the right height for your child.
